1. Field of the Invention
The present invention generally relates to a chemical compound capable of emitting visible light and having electrical conductivity, a luminescent material that contains the chemical compound, an electro-luminescent device including a luminous layer that contains the chemical compound, and a display apparatus that includes a plurality of the electro-luminescent devices.
2. Description of the Related Art
It is expected for an electro-luminescent (EL) device to be utilized as display device elements for various display apparatuses or a light source for illumination such as a backlight, since luminescence with high brightness can be obtained from the EL device when even a low voltage is applied to the EL device. The EL device elementarily has a structure provided by laminating a transparent anode, one or more (typically, 3 through 5) layers that include a luminous layer, and a cathode from bottom to top, on a transparent glass or plastic substrate. When a voltage is applied between the anode and the cathode of the EL device, both holes from the anode and electrons from the cathode are injected into the luminous layer. Herein, the luminous layer contains a luminescent material that emits light by utilizing energy provided due to the recombination of the holes and the electrons.
Additionally, the plurality of the layers may include not only the luminous layer but also a hole-transporting layer for improving the injection efficiency of the holes or an electron-transporting layer for improving the injection efficiency of the electrons, or both of them that sandwich the luminous layer therebetween.
Also, an electro-luminescent (EL) display apparatus is one type of image display panel, in which a plurality of the above-mentioned EL devices are arranged, for forming an image by driving the EL devices as the pixels of the image. Particularly, in order to produce a full-color image display panel, it is necessary to arrange EL devices that emit light with one color of the three primary colors in the additive color process, that is, red, green, and blue lights, as the pixel of an image.
Now, the following three approaches for realizing the EL devices for emitting one of the three-color lights as described above have been suggested.
As the first approach, the luminous layer of the EL device that constitutes each pixel of an image and emits red light, green light, or blue light is formed from a luminescent material that is selected independently for each color and is different from the luminescent material for other colors. Each of the luminescent materials can generate red light, green light, or blue light, by utilizing energy provided due to the recombination of the holes and the electrons. Thus, the full color EL display apparatus having three-color pixels of an image can be produced.
As the second approach, first, the luminous layers of all the EL devices corresponding to all the pixels of an image are formed from one kind of luminescent material for emitting white light. The luminescent material for emitting white light emits white light that contains red light, green light, and blue light, by utilizing energy provided due to the recombination of the holes and the electrons. Secondly, color filers for transmitting red light, green light, or blue light are disposed on the surfaces of the EL devices as the pixels, through which white light is emitted. Thus, each of the three-color lights is extracted from the white light emitted from the EL device through the color filter for the corresponding color and the full color EL display apparatus having three-color pixels of an image can be produced.
As the third approach, all the EL devices corresponding to all the pixels of an image are formed from a luminescent material for emitting blue light. For pixels that emit red light or green light, a color conversion layer for converting blue light into red light or green light, respectively, is formed on the surface of the EL device that emits blue light. The luminescent material for emitting blue light generates blue light with comparatively high energy by utilizing energy provided due to the recombination of the holes and the electrons. Both color conversion layers for red light and for green light are photo-excited by the blue light with comparatively high energy generating from the luminescent material for emitting blue light and emit red light and green light, respectively. Thus, the full color EL display apparatus having three-color pixels of an image can be produced, from which the green light and the red light that are converted from the blue light as well as the blue light are emitted.
In the above-mentioned first approach that is most widely used at present, a luminescent material for one luminous color generally has a molecular structure quite different from luminescent materials for other colors. Accordingly, where the hole-transporting layer and/or the electron transporting layer are/is laminated on those luminous layers having molecular structures quite different from each other, the conditions for laminating a hole transporting layer and/or the electron transporting layer on the luminous layer for one color are also quite different from the conditions for other colors, dependent on the kinds of the luminous layers for respective colors. Thus, it is generally difficult to produce an EL display apparatus by employing such luminescent materials having molecular structures quite different from each other for respective colors.
As described above, since the molecular structure of a luminescent material for one color is quite different from that of luminescent materials for other colors, the service life of the luminescent material for one color is also different from service lives for the luminescent materials for other colors. That is, deterioration of the pixels for one color of the EL display apparatus is different from that of the pixels for other colors. Herein, the anodes and/or the cathodes of the EL display devices are usually common in some EL devices. Accordingly, EL devices corresponding to respective pixels are dependent on each other in the EL display apparatus, so that it is difficult to replace the EL device with a new one in one-pixel units. Thus, when the above-mentioned luminescent materials for respective colors with a service life quite different from each other are employed, it is difficult to replace an EL device for a luminous color that is made from a luminescent material with short service life with a new one independently, and there is included the disadvantage that the whole of the EL display apparatus has to be replaced with a new one.
As described above, when the EL display apparatus is produced according to the first approach, both the cost for producing the EL display apparatus and the cost for replacing the EL display device with a new one become high.
In the above-mentioned second approach, a common luminescent material for emitting white light is employed for the luminous layers of all the EL devices, so that the lamination conditions and the service lives for the luminescent materials are basically common among all the EL display devices. However, in the second approach, since the color filter acts to extract light with a wavelength in a specific spectral region from the white light, visible light other than the extracted light with the wavelength in the specific spectral region may be absorbed by the color filter. Accordingly, the efficiency of the utilization of the visible light relative to the generation of the white light is low and the loss of the energy of the white light is high.
Also, in the above-mentioned third approach, although a common luminescent material for emitting blue light is employed for the luminous layers of all the EL devices, for the EL devices for emitting red light or green light, the color conversion layers for red light or green light have to be laminated, so that the EL devices for emitting light with colors different from each other have to be produced in lamination structures different from each other.
Also, in the second approach, even if the intensities of the white lights emitted from the EL devices are approximately common, after the white lights transmit through the respective color filters provided on the EL devices, the intensities of the transmitted lights for the respective colors are generally different from each other. Also in the third approach, even if the intensities of the blue lights emitted from the EL devices are approximately common, generally, the intensities of both the green light and the red light emitted from the respective color conversion layers are quite different from the intensity of the blue light that is not converted by a color conversion layer. In addition, since the color conversion layer for red color is formed from a material different from that for the color conversion layer for green color, the quantum yield of the color conversion layer for red color is usually different from the quantum yield of the color conversion layer for green color, so that the intensity of the green light emitted from the color conversion layer for green color is different from the intensity of the red light emitted from the color conversion layer for red color. Accordingly, when an EL display apparatus is produced according to the second or third approach, it is necessary to control the luminous intensity for each color. However, it is generally difficult to greatly change the voltage applied between the anode and the cathode of the EL device for each color.